This invention relates generally to receiving systems and, more particularly, to receiving systems adapted to receive energy and having signal processors used to extract information from said received energy.
As it is known in the art, systems such as, for example, communication systems, radar systems, sonar systems, and the like have a receiver which is used to detect the presence of energy and a signal processor which is used to extract information from the detected energy. In particular, with radar systems and sonar systems, the receiver is used to detect energy reflected from an object and extract information relating to the object from which the received energy was reflected. A problem common to radar systems and sonar systems is detecting the energy in the presence of noise and clutter (radar)/reverberation (sonar) and extracting information from the detected energy with minimal loss due to noise and clutter/reverberation.
As it is also known in the art, radar systems rely upon the transmission of radio frequency energy in a propagating medium typically air to detect, map, or otherwise obtain information about a region in which the radar system is deployed. In particular, uses for radar include the detection, tracking, and identification of targets such as other radar systems, as well as objects travelling through the region covered by the radar system.
As it is also known in the art, sonar systems rely upon the transmission of sonic energy in underwater environments to detect, map, or otherwise obtain information about the region in which a sonar system is deployed. In particular, one of the uses for sonar is the detection and recognition of targets. Particular targets of interest for military vessels such as submarines, mine sweepers, and ships are mines and other underwater explosive type of devices, as well as other vessels such as other submarines.
A problem is encountered in using sonar to detect and recognize small targets such as mines and submarines particularly at long distances. Since sonar operates using acoustic waves to recognize such targets, it is necessary to obtain high fidelity images of the targets of interest by processing echos or reflections of acoustic energy. The representations of the targets such as mines and submarines should be of sufficiently high fidelity to permit such representations to discriminate against other non-target contacts such as topographic features on the bottom of the ocean floor, for example. Target imaging for such feature extraction is generally done with high frequency sonar. One problem, however, with using high frequency sonar is that absorption losses in water for high frequency acoustic energy are significant. High frequency acoustic signals are attenuated rapidly in ocean water thus mitigating against their use for long range detection and identification of small objects.
Conventional sonar systems perform tasks such as long range detection and recognition, often transmit sonar signals of lower than ideal frequency for the task of detecting and recognizing a target in order to acquire a potential target. Also, tracking of a target and recognition of a target may occur using a different sonar mode operating at high frequencies. In any event, received acoustic energy is processed by a sonar receiver to extract some information relating to the object from which the received echo energy originated.
Conventional receiving systems such as communication, radar, and sonar systems employ so-called matched filter processing to extract information from the received signal. In matched filter processing, generally energy such as electromagnetic energy for communication and radar systems or acoustic energy for sonar systems is transmitted having a known shape, pulse rate and frequency spectrum. Conventional receiving systems such as communication systems, radar systems, and sonar systems employ so-called matched filter processing in the receiver to extract information from a received signal. In systems such as communication and radar systems, generally electromagnetic RF energy is transmitted by a transmitter having a known shape, pulse width, and frequency spectrum, whereas for a sonar system acoustic energy having a known shape, pulse rate, and frequency spectrum is projected from a sonic projector. In radar and sonar systems, a portion of the transmitted energy is reflected from an object or target and a further portion of the energy is intercepted by an antenna (for radar systems) or hydrophone (for sonar systems). In communication systems, a portion of the transmitted energy is intercepted directly by an antenna and coupled to the communications receiver. In either event, since the transmitting system transmits energy having a known shape, pulse rate, and frequency spectrum, the transmitted characteristics of the transmitted energy can be filtered out or removed from the received energy by employing a matched filter. The matched filter has a filter response corresponding to the complex conjugate of the transmitted spectrum of the signal.
For sonar systems, acoustic energy from objects are received by a sonar hydrophone which converts the echo acoustic energy into electrical signals having a particular signal shape or waveform as well as frequency. The signals from the acoustic hydrophone are fed ultimately to a matched filter.
By filtering such received acoustic or electromagnetic energy with a matched filter, the filter effectively removes the spectrum of the transmitted signal leaving behind information relating to the acquired object. Matched filtering is a useful technique provided that signals of appropriate frequency can be transmitted and received from an object. Theoretically, an ideal matched filter processor provides a receiver having the highest signal to noise ratio.
Match filtering is thus employed in both RF (radar and communication) and acoustic (sonar) applications. However, several problems exist with match filtering particularly for acoustic processing in a underwater environment. For sonar systems, the processes that contribute to the formation of echos from underwater objects and targets are complex. For example, targets in an underwater environment are relatively rigid in comparison to high frequency sonar wavelengths and thus at high frequencies targets act as reflectors. However, at low frequencies where targets are more nonrigid, targets cause signal dispersion. Further, the propagation environment typically nonlinearly attenuates the acoustic energy as a function of frequency. This attenuation is particularly severe at high frequencies. Further, propagation characteristics of such acoustic waves are also affected by water depth, water temperature, and topographic features of the area. Thus, it is difficult to provide a match filter which would remove the transmitted spectrum of the signal, as well as compensate for changes introduced into the signal as a result of the propagation medium and non-ideal effects of the target.
Moreover, detection of underwater objects such as submarines and mines generally occurs against a background of clutter, such as immovable objects including the surface of the sea floor. In sonar systems, this background causes reverberations or multiple echos or reflections of the acoustic energy from such objects.
Mines and other such devices which are targets of interest for the sonar may also be buried on the sea floor bottom. While relatively low frequency acoustic signals can penetrate the sea floor, low frequencies will not provide echo returns which could be processed into images of high fidelity. While high frequency acoustic signals can provide echo returns which can be processed into high fidelity images, high frequency acoustic signals cannot sufficiently penetrate the sea floor. Accordingly, buried objects such as mines present a further problem concerning long range detection with minimum false alarm (or false detection) rates of occurrence.
Thus, long-range detection of small objects by conventional sonar systems employing matched filtering is fraught with many problems. It is desirable, therefore, when using matched filtering, to operate using lower frequencies having lower propagation losses to provide good long range detection. However, good long range recognition is difficult to provide with matched filtering techniques since lower frequencies will not provide images having sufficient fidelity or resolution to be recognized. Thus, although long-range detection is possible with low frequency sonar, long-range recognition of the detected object and discrimination of a true target from a false target is difficult.
Similar considerations also exist for RF systems such as radar and communication systems. For example, often radar signals must detect objects against a background of clutter. Further, multipath echoes can often affect receiver performance.
In general, although theoretically matched filter processing should offer the highest detectability for any received energy from a known transmitted signal, the presence of clutter and other media corrupting influences on the received energy make it difficult to design a receiver having a matched filter response which can not only remove the transmission spectrum from the received signal but also compensate for effects on such signals caused by clutter, reverberation, multipath, and other effects on the signal by the propagating median.